1. Field of the Invention
The present invention relates to a color-drift-error correcting method by controlling respectively different color-image forming operations so as to minimize a color-drift error. The present invention also relates to an image forming apparatus such as copying machines, printers, facsimile machines, complex machines and the like, being provided with a controlling unit that controls respectively different color image forming units so as to minimize the color-drift error.
2. Description of the Related Art
In the field of color image forming apparatus, there has been known a tandem-type image forming apparatus for forming a color image, having a plurality of image carriers such as photoconductors arranged along an endless transfer belt, which is a transfer medium. In the tandem-type image forming apparatus, an electro-photographic process such as development is performed while an electrostatic lateral image is formed for each of yellow (Y), magenta (M), cyan (C), and black (K) on a respective image carrier. These images of different colors on the respective image carriers are superposed one another to finally form a color image on a transfer sheet. In the tandem-type color image forming apparatus, it is necessary to accurately position toner transfers of different color toner images to form a high quality image. If there is a positional drift between different color toner images, a high quality color image will not be obtained due to color drift between superposed toner images formed on the transfer sheet.
FIG. 22 is a view for explaining color drift in a vertical scanning direction. FIG. 23 is a view for explaining a skew drift. As such color drift, vertical (sub scanning direction) scanning registration drift which is deviation in the vertical scanning direction from an ideal image formation line (position at which toner image is to be properly formed) indicated by the bold line in FIG. 22, and skew drift, which is inclination in the vertical scanning direction from an ideal image formation line indicated by the bold line in FIG. 23, are exemplified. In view of the above, an apparatus has been proposed in which after forming a mark-pattern for positioning, the position of the mark-pattern is detected by a sensor, and being based on the detection result, the position is corrected by matching drifted image formation line with the ideal image formation line. That is written for example in Japanese Patent application Laid-open No. 1-142671 and JP Patent No. 33514352.
In the patent document mentioned above it is written that a mark-pattern is formed on each end of an intermediate transfer belt. Each mark-pattern includes a series of mark images of different colors of toner arranged in a moving direction of the belt. Each of these mark images of each mark-pattern is detected by a sensor, and a drift amount between the detected position and an ideal position is calculated, and a toner transfer position is corrected.
FIG. 25 is a view for explaining an amount of color drift compared to the ideal image after the color drift is corrected based on the former art. Due to positioning errors of lens, mirror and the like optical devices in an optical writing device, or due to relative positioning errors between an image carrier and an optical writing device, a trajectory of scanning line of laser beam sometimes curves. In such a case, an image formation line in the horizontal scanning direction (main scanning direction) curves like an arch as shown in FIGS. 24A and 24B. In the methods disclosed in the patent documents mentioned above, mark-patterns are formed on both ends of an intermediate transfer belt, and position correction is conducted such that both ends of an image formation line match with the ideal image formation line. Therefore, when the image formation line in the horizontal scanning direction curves in the vertical scanning direction as described above, the middle part of the image formation line after correction will be largely deviated from the ideal image formation line indicated by the bold line in the drawing as shown in FIG. 25. As a result, color drift occurs-in the middle part in the horizontal scanning direction of image, and a high quality color image is-not obtained.